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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made August 23, 2017

 Days 3-7Days 8-14Prob. Days 8-14
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Categorical Outlooks
Experimental Probabilistic Outlooks (Information)

Valid Saturday August 26, 2017 to Wednesday September 06, 2017

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 23 2017

Synopsis: Tropical Depression Harvey is forecast to intensify and then move inland into southeast Texas on August 26. Its remnant low is expected to remain across the western Gulf Coast region through early next week. A tropical cyclone may develop near the Southeast coastline early next week with a subsequent track to the northeast. Upper-level high pressure is forecast to build across the western U.S. during the final week of August. Upper-level low pressure is likely to remain centered over western Alaska and the Aleutians through Week-2.

Hazards Detailed Summary

For Saturday August 26 - Wednesday August 30: As of 2pm EDT on Aug 23, Tropical Depression Harvey is located over the Bay of Campeche. Harvey is likely to intensify prior to this period and could become a hurricane when it makes landfall along the Texas Gulf Coast later this week. Based on the latest model solutions, high winds (potentially hurricane force) and storm surge are forecast along the Texas Gulf Coast through Aug 26. After making landfall, the remnant low is expected to move erratically across the western Gulf region. The 6Z GFS and 0Z ECMWF ensemble means are in better agreement today, maintaining a remnant low near the western Gulf Coast through early next week. Therefore, a prolonged period of heavy rainfall and flooding is likely for the Texas Gulf Coast and lower Mississippi Valley through August 30. The deterministic 0Z ECMWF model indicates 7 to 10 inches, locally more, of rainfall from Aug 26 to 30 across the outlined heavy rain area. Maximum rainfall amounts may exceed 15 inches across southeast Texas and southwest Louisiana with an increasing risk of major flooding. Please refer to the National Hurricane Center and local NWS offices for the latest updates and forecasts.

A trough of low pressure and enhanced moisture are expected to bring heavy rain (locally more than 2 inches) to parts of central and south Florida on Aug 26. Deterministic model solutions continue to indicate slow tropical or subtropical cyclone development offshore of the Southeast coastline this weekend. An increasing pressure gradient between this area of low pressure and a 1028-hpa surface high over New England is expected to result in high winds (gusts above 35 knots) across the Outer Banks on Aug 28 and 29.

An upper-level ridge is forecast to strengthen across the western U.S. with 500-hpa heights increasing to more than 588dm early in this period. Much above-normal temperatures are posted for areas of the Pacific Northwest, northern Great Basin, and northern Rockies where maximum temperatures are forecast to average around 12 degrees F above normal and exceed 90 degrees F. Also, numerous wildfires continue to burn in these same areas with the largest wildfire (more than 68,000 acres burned) at Nena Springs in northern Oregon.

Anomalous southerly flow with a tropical connection is expected to affect southern mainland Alaska next week as a highly amplified trough (ridge) develops over the Aleutians (eastern Alaska). The anomalous southerly, onshore flow is likely to bring heavy rain (more than 1.5 inches per 24 hours) from the Alaska Peninsula east to the Prince William Sound. The 0Z deterministic ECMWF model indicates a 72-hour maximum of more than 10 inches along the southern Kenai Peninsula from Aug 28 to 30.

For Thursday August 31 - Wednesday September 06: The remnant low from Tropical Depression Harvey is expected to lift away from the Gulf Coast at the beginning of Week-2. Although there is support for an area of heavy rain somewhere across the eastern U.S. on day 8, the exact location varies largely among model guidance. The 0Z ECMWF ensemble mean indicates generally less than 0.25 inch across much of the eastern U.S. on Aug 31, representative of the large ensemble spread. Also, the area for heaviest rain varies among recent GFS ensemble mean solutions. Therefore, a categorical heavy rain hazard is not posted beyond day 7.

The GFS and ECMWF ensemble means maintain a stable longwave pattern through the beginning of September with an upper-level ridge (trough) over western (eastern North America). The most anomalous longwave trough across the Northern Hemisphere remains the upper-level trough extending from the Bering Sea to south of the Aleutians. The amplified upper-level ridge supports a slight (Aug 31-Sep 6) to moderate (Aug 31) risk of much above-normal temperatures across the western U.S.

Tropical cyclone (TC) development is favored across the east Pacific by the end of August. A northwest track of a TC close to the Baja peninsula is expected to bring a surge of moisture north from the Gulf of California into the desert Southwest during early September. Therefore, a flash flooding possible hazard is posted for the desert Southwest.

The U.S. Drought Monitor valid on Aug 15 indicates that severe to exceptional (D2-D4) drought coverage decreased slightly from 5.38 percent to 5.33 percent across the continental U.S.

Forecaster: Brad Pugh

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Please consult local NWS Forecast Offices for short range forecasts and region-specific information.